The present invention relates to a solid-state image pickup device formed with an image pickup element having a plurality of unit pixels arranged therein and a driving control method for the solid-state image pickup device, and more particularly to a technique for discharging unnecessary charge in a solid-state image pickup device of a MOS type, a CMOS type or the like in which a pixel signal from a unit pixel is read by address control.
As solid-state image pickup devices of a system that reads pixel signals from charge generating sections including a plurality of photoelectric converting elements (photodiodes or the like) within an image pickup unit by controlling a pixel position by address control, there are solid-state image pickup devices of a MOS type and a CMOS type (which will hereinafter be typified by the MOS type for description unless otherwise specified). For example a solid-state image pickup device having unit pixels disposed in a form of a two-dimensional matrix is referred to as an X-Y address type solid-state image pickup device.
The address type solid-state image pickup device uses MOS transistors as a switching element for selecting a pixel and as a switching element for reading signal charge, for example. MOS transistors are also used for a horizontal scanning circuit and a vertical scanning circuit, thus having an advantage of being able to be fabricated in a series of structures with the switching elements.
Each unit pixel in the MOS type solid-state image pickup device, for example, is formed with MOS transistors, and is configured to output a signal charge accumulated in the pixel by photoelectric conversion to a pixel signal generating section, convert the signal charge into a current signal or a voltage signal, and then output the current signal or the voltage signal.
The pixel signal generating section provides an output signal in substantially linear relation to an amount of charge accumulated in the unit pixel by photoelectric conversion. An amount of charge that can be accumulated in the unit pixel determines a dynamic range of the image pickup element, or the dynamic range of the image pickup element is determined by a saturation signal amount of the pixel and a noise level.
Some MOS type image pickup devices of the X-Y address type having conventional unit pixels arranged two-dimensionally in the form of a matrix reset (discharge) unnecessary signal charge from a row of pixels to a signal line in a horizontal blanking period when signal charge is not read, for example, to perform electronic shutter operation.
The shutter speed of an electronic shutter, that is, a time corresponding to an accumulation time of a pixel is determined by a period from a point in time of discharge of signal charge to a point in time of reading of signal charge. Hence the accumulation time differs between the right and the left in a horizontal direction. That is, the accumulation time of a pixel outputted by a horizontal scanning pulse varies in proportion to timing of output of the pixel.
The difference in the accumulation time can be ignored when the shutter speed is slow and the accumulation time of the pixel is set sufficiently long. However, when the shutter speed is set fast to a degree that the shutter speed is not so different from a horizontal scanning period of the shutter speed, the difference in the accumulation time appears as shading in a line direction (horizontal direction) in an image, thus presenting a problem.
In order to solve this problem, a structure has been proposed to realize a function referred to as a global shutter function in which the exposure accumulation time of each pixel is constant (simultaneous exposure) when the electronic shutter operation is performed. For example, a structure has been proposed in which a charge storage section is provided between a charge generating section and a pixel signal generating section in each pixel, so that after simultaneous exposure of all pixels to light, a signal charge generated in the charge generating section is transferred simultaneously to the charge storage section (see for example U.S. Pat. No. 5,986,297).
In this system, a signal charge generated as a result of light entering a photoelectric converting element of the charge generating section is transferred to the charge storage section simultaneously in all the pixels and temporarily stored in the charge storage section. The signal charge is sequentially converted into a pixel signal in predetermined readout timing. Also, in this system, a charge accumulated in the charge generating section as a result of light entering the photoelectric converting element of the charge generating section after the transfer is discharged prior to a next exposure accumulation.
Thereby, a pixel signal corresponding to the amount of signal charge stored in the charge storage section is obtained, and the electronic shutter function without a difference in exposure accumulation time can be realized by adjusting timing of transfer to the charge storage section after exposure.
However, when intense light enters the photoelectric converting element of the charge generating section, a so-called “blooming phenomenon” becomes a problem, in which phenomenon a charge more than a maximum amount of charge that can be accumulated in the photoelectric converting element is generated, and the charge overflows the photoelectric converting element through a transfer gate or a channel stop region into the pixel signal generating section or a charge generating section within an adjacent pixel or the like. The blooming phenomenon also occurs in the structure for realizing the global shutter function by transferring a signal charge obtained in the photoelectric converting element to the charge storage section simultaneously in all the pixels, for example.
When the blooming phenomenon occurs, a white band-shaped or white circle-shaped pattern is observed in a picked-up image, degrading picture quality. In the structure for realizing the global shutter function with the charge storage section, in particular, an excess charge generated in the charge generating section overflows into the charge storage section within its pixel. Since a pixel signal is obtained according to an amount of signal charge stored in the charge storage section, as described above, the blooming phenomenon varies a signal component of the pixel itself. Furthermore, when a charge generated as a result of light entering the photoelectric converting element after a signal charge is transferred to the charge storage section overflows into the charge storage section, a problem (quasi-blooming phenomenon) similar to the blooming phenomenon occurs.